1. Field of the Invention
This invention relates to the field of loading liquefied gases into cylinders.
2. Prior Art
Filling stations are used to vaporize liquid cryogenic substance for filling high pressure industrial and medical gas storage devices such as cylinders. The liquefied gas is stored in large storage tanks to periodically refill cylinders with high pressure gas which are then transported to the place of use.
Prior to the filling of the cylinders, the cylinders are evacuated to remove any remaining substance or contaminants from them. During a typical evacuation cycle a cylinder is first vented to atmospheric pressure and then, by means of a vacuum pump, drawn down to a very low pressure. However, a small amount of the substance may still remain in the cylinder after such an evacuation. If the cylinder contained a contaminant then the contaminant will be in the cylinder after filling. This is very undesirable because in many applications a very high standard of purity is required. Thus, it was known to try to determine whether there were any contaminants in the cylinder before filling and to perform additional evacuation cycles on the cylinder until the contaminant is removed. A prior art method of determining whether there were contaminants in the cylinder was to open the valve on the cylinder and sniff the gas released prior to evacuation. This was very undesirable because it was dangerous to the operator since unknown contaminants may have been in the cylinder. Additionally, it was dangerous because some of the product gases could cause an explosion. Additionally, this "sniff test" was not very accurate. Typically, even experienced operators could not detect contaminants in a lower concentration than approximately fifty to one hundred parts per million. For some applications, this level is not satisfactory. Additionally, the operator could only detect contaminants that had an odor. Thus odorless gases could not be detected at all using this method.
In another aspect of filling, after evacuation, substance was loaded into the cylinders using a pump which withdrew substance from the storage container and forced it into the cylinders. These pumps were subject to cavitation in which gas bubbles within the pump provided less resistance to the pump than the liquid. The lower resistance caused the pump to move faster. As the pump moved faster it heated causing additional gas bubbles to form thereby providing positive feedback to the cavitation process. This cavitation process could damage or even destroy the pump. Thus it has been known in the prior art to try to prevent cavitation as well as to try to detect cavitation as quickly as possible and shut the pump down.
Additionally, in prior systems substance was not permitted to flow into the pump to cool the pump down until the evacuation of the cylinders was complete. Thus a time delay was incurred while waiting for the cool down process to be completed.
Even if the pump was sufficiently cooled down prior to starting the pump motor, cavitation could still occur later during the loading of the cylinder. One commonly used method in the prior art for preventing cavitation from damaging pumps was simply to manually shut down the pump when the operator heard banging noises from the pump indicating that the pump was entering cavitation. This method was unsatisfactory because even short periods of banging caused premature wearing of the pump. Additionally, if the operator was busy or had left the area and no one was there to hear the pump banging the pump could be destroyed.
One method known in the prior art for dealing with this problem was to monitor the pressure differential across the pump. If the difference between the pressure at the inlet of the pump and pressure at the outlet of the pump was not within a predetermined range, it could be determined that the pump was in cavitation or about to go into cavitation and the pump could be automatically shut down. However, this differential pressure method of detecting cavitation, while adequate for centrifigal pumps, was not a reliable way to detect cavitation in reciprocating pumps because of the different geometry of reciprocating pumps. Additionally, the pressure differential method was not reliable for higher pressure pumps and many applications could not be reached using lower pressure pumps.
It is also well known in the art of loading liquefied gases into cylinders to load a plurality of cylinders at one time using a single large manifold. In this method, the liquefied gas is supplied to the manifold and the various cylinders to be filled are attached to the manifold. It is also known to provide liquefied gas to a plurality of manifolds simultaneously wherein several cylinders may be coupled to each manifold. In many commercial applications it is desirable to load several different sizes of cylinders simultaneously. However, if different size cylinders are attached to a manifold at the same time, the smaller ones fill before the larger ones resulting in inefficiency. In the past determination was made when the smaller ones were full and the individual valves for the filled cylinders were closed to prevent additional substance from flowing into them while permitting substance to continue to flow into the larger cylinders. This was inaccurate because during the time required to close the valve of one of the smaller cylinders, substance continued to flow into other smaller cylinders. Thus the smaller cylinders would all be loaded with different amounts of substance. Furthermore, this method required an operator to be present and attending the cylinders until each was filled and then to close the valve.
In a still further aspect, after filling, the cylinders must be analyzed to determine whether they meet the specified level of purity. Testing of a single cylinder on a manifold is sufficient to test all the cylinders on the manifold. To perform this test, one of the cylinders was disconnected from the manifold and, in typical operations, placed on a cart and wheeled to a different location to be coupled to an analyzer. The analyzers were typically in a different location from the manifold because of the large size of these various pieces of equipment. This process was time consuming and laborious. In addition, since there are many different types of product gases as well as different cylinder pressures it was necessary to be careful to hook the cylinder up to the correct analyzer. Accidentally coupling a cylinder up to an incorrect analyzer may destroy the analyzer.